1. Field of Invention
The present invention relates to system and method for compensating friction due to rack bending in a motor driven power steering (MDPS) system, and more particularly to system and method for compensating friction due to rack bending in a motor driven power steering system which can improve steering comfort by compensating excessive friction between a rack bar and a bearing with motor torque when the rack bar is bent by a lateral force in a vehicle.
2. Description of Related Art
Power steering systems for vehicles are systems for increasing/decreasing the force that turns steering wheels and hydraulic power steering has been generally used.
Vehicles that are recently coming into the market are equipped with a motor driven power steering (MDPS) system to change a steering force in accordance with the running speed.
Thee motor driven power steering system is a speed sensitive power steering system, in which an Electronic Control Unit (ECU) of a motor driven power steering module provides a driver with the optimum steering comfort by controlling the steering angle and the operation of a motor in accordance with the speed of a vehicle.
In more detail, the motor driven steering system, a system that assists steering power with the power from a motor, includes a motor mounted on a steering column and generating power for steering, a rack bar connected with a steering shaft, a reduction gear box connecting the output shaft of the motor and the rack bar so that power can be transmitted therebetween, and an ECU controlling the motor in response to input about the vehicle speed, and assists a steering force.
As shown in FIGS. 1A and 1B, a rack bar 12 is arranged to be movable to the left or right through a bearing 14 in a rack bar housing 10. The rack bar 12 moves left or right for steering with rolling friction on the bearing 14 when a vehicle turns.
As shown in FIG. 2, a lateral force perpendicular to the steering shaft 16 is generated when the vehicle turns, and it is transmitted to the steering shaft 16 and the rack bar 12, as shown in FIGS. 1A and 1B.
With the lateral force transmitted to the rack bar 12, the rack bar keeps horizontal without bending when its rigidity is sufficient, but the rack bar bends when its rigidity is not sufficient.
As a rack bar is bent by a lateral force, excessive friction is generated on the contact surface between the rack bar and the bearing in the rack bar housing, such that smooth handle is not provided.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.